1. Field of the Invention
This invention relates to a method and system for monitoring laser beam welding and more particularly, to a method and apparatus for monitoring the behavior of a laser beam during welding.
2. Description of the Prior Art
Laser beam welding is widely practiced nowadays because welding beam input (energy) can be applied to only a desired weld zone and little thermal influence is imposed in the vicinity thereof. In this regard, to finely complete welding, the welding beam input and resultant depth of penetration of welding must be controlled properly.
It is generally difficult in laser beam welding to directly detect the welding beam input being actually applied to a workpiece. Therefore, as shown in FIG. 11(a), a part of a laser beam 15 generated by a laser oscillator 1 is guided by a quartz plate (semi-transparent mirror) 26 to a photodiode 6 such that the detection output of the photodiode 6 indicates the (laser beam) output of the laser oscillator 1. Conventionally, the behavior of welding is deduced from monitoring the laser beam output of the oscillator.
However, in the foregoing conventional system wherein only the output of the laser oscillator is detected, if the transmission loss of an optical system for transmitting the laser beam from the laser oscillator to a weld zone increases suddenly during welding due to the degradation of optical parts, for example, a large discrepancy arises between the laser beam output and the welding beam input being actually applied to a workpiece; therefore, accurate monitoring cannot be attained as illustrated in FIG. 11(b). Further, even if welding becomes unstable and the depth of penetration is decreased due to a change in the welding conditions, this causes no change in the output of the laser oscillator; therefore, the behavior of the welding beam cannot be properly determined by an operator.
A different system is known in which the behavior of laser beam welding is deduced from monitoring infrared radiation at a weld zone. Specifically, U.S. Pat. No. 4,443,684 (corresponding to Japanese Patent Laid-Open No. 57-124586) discloses a system applicable in the field of laser beam processing, inclusive of welding, which uses a carbon dioxide gas laser as a laser beam source and sends the infrared radiation at a spot under laser beam processing to a detector to obtain the temperature of the spot.
In this U.S. patent, an infrared ray having a wavelength of 10.6 .mu.m is used and the temperature of a weld zone is measured. However, since the infrared ray reaching the detector includes a reflected part of the laser beam and laser plumes appear in the vicinity of a molten pool to cause much infrared radiation, it is difficult to accurately detect the temperature of the weld zone. Further, since the temperature of the weld zone generally does not indicate the welding beam input and the depth of penetration, it is difficult in laser beam welding to monitoring the welding beam input and the depth of penetration on the basis of the detected value of temperature.